Make the meringue: whisk the egg whites until stiff, add half the sugar, whisk again until stiff, then repeat with the remaining sugar. Mix the coffee with 3 tbsp boiling water to dissolve the granules. Gently fold the walnuts and 1 tsp of the coffee into the meringue. Set aside while you make the cake mixture.
Heat oven to 180C/160C fan/gas 4. Grease 2 x 20cm loose-bottomed sandwich tins and line the bases with baking parchment. Beat together the butter and sugar until light and fluffy, then gradually whisk in the eggs. Fold in the flour and baking powder, then finally fold in 2 tsp of the coffee liquid. Divide the mixture between the 2 tins, putting a little more mixture in one than the other.
Spread the meringue on top of the tin containing less cake mix. Bake the untopped sponge for about 20 mins until an inserted skewer comes out clean. Continue baking the meringue-topped sponge for an additional 20 mins, then carefully remove both cakes from their tins and leave to cool on a wire rack.
To make the filling, beat the butter and icing sugar together, adding 2 tsp more coffee. To make the icing, mix a final tsp of the coffee into the icing sugar. Add a drop more water to make a runny icing. 5 Sandwich the 2 cakes together with the coffee filling, putting the meringuetopped sponge on top. Drizzle all over with the coffee icing to finish.